In current wireless telecommunication systems such as GSM and UMTS Terrestrial Radio Access Network (UTRAN), the possible downlink frequencies are enumerated where each combination of operating frequency band and frequency is given a unique number called Absolute Radio Frequency Channel Number (ARFCN). The ARFCN is a function that maps a number, e.g. 42 to a frequency and to a band. For example 42 is mapped to band 1 and carrier frequency 2134.6 MHz by the ARFCN.
Since some bands are overlapping the same frequency may actually be assigned multiple numbers, one for each band.
In frequency division duplex (FDD) mode of operation used in UTRAN FDD, the uplink and downlink transmission take place on different carrier frequency channels, as described in 3GPP TS 25.101: “UE Radio transmission and reception (FDD)”. The carrier frequency channel spacing in UTRAN FDD in each direction is 5 MHz. Therefore, in the FDD mode both uplink and downlink transmission can occur simultaneously in time.
On the other hand in time division duplex (TDD) mode used in UTRAN TDD, the uplink and downlink transmission take place on the same carrier frequency channel but in different time slots or time frames, as described in 3GPP TS 25.102: “UTRAN (UE) TDD; Radio transmission and reception”. The carrier frequency channel spacing in UTRAN TDD is also 5 MHz.
The half duplex that is used in GSM can be regarded as a hybrid scheme where the uplink and downlink are transmitted on different carrier frequencies and on different time slots as described in 3GPP TS 05.05: “Radio Transmission and Reception”. This means that uplink and downlink transmission do not occur simultaneously. The carrier frequency channel spacing in GSM is 200 KHz.
In any of the above schemes, in principle the base station may be utilizing more than one frequency channel to transmit and receive.
In one scenario the base station may provide only unicast services, which can use transmit and receive channels on different or multiple frequency channels. Generally both transmit and receive channels belong to the same frequency band. The unicast services are user specific and bi-directional.
In another scenario the base station provides only Multimedia Broadcast (MBMS) over single frequency networks (MBSFN), which generally uses unidirectional downlink channels. However, there can be multiple broadcast services, where each broadcast service is carried on a different frequency channel either within the same or a different frequency band.
In a hybrid service scenario, the base station can offer unicast and broadcast services on different frequency channels, which in turn may belong to the same or different frequency bands.
In the hybrid service scenario in E-UTRAN (Evolved UTRAN) it is required that the User Equipment (UE) is able to simultaneously receive frequency channels carrying unicast and broadcast services, as disclosed in 3GPP TR 25.913: “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)”.
In some situations, the network requests the UE to perform handover to another frequency. Therefore, the UE must be made aware of the new frequency channels. For instance two frequency channels, one for transmission and the other for reception when it concerns a unicast service in FDD based systems.
When sending commands to UE to make the UE change operating frequency the unique number (ARFCN) is sent.
In the FDD mode the uplink frequency channel number can easily be derived from the ARFCN of the downlink frequency (or vice versa) since the ARFCN maps to an operating frequency band and an operating frequency. For each band the duplex distance, which is the allowed difference between uplink and downlink frequency, is fixed and a simple calculation then gives the uplink frequency.
For various reasons, e.g. varying channel allocations, different up and downlink bandwidths, there is a need to signal the uplink frequency to the mobile equipment as well, since for a certain downlink frequency there may be many possible uplink frequencies where only one should be used.
For instance in E-UTRAN FDD the duplex distance is not fixed. This means that the frequency channel number or the ARFCN used on the uplink cannot be uniquely derived by the UE from the downlink ARFCN signaled by the network or vice versa. In the state of the art technology, the network has to signal the entire ARFCN for both uplink and downlink. This will increase the signalling overheads by a factor of two since both uplink and downlink ARFCN are to be signaled. As an example, if 15 bits are used for signalling one ARFCN e.g. the downlink frequency channel then 15 bits would also be needed for signaling the uplink frequency channel.
There is also a requirement in E-UTRAN that it must be able to provide unicast and broadcast services on different carrier frequencies, which in turn should be simultaneously receivable by the UE. In such scenarios, in the current technology, the network will have to signal the ARFCN numbers of all the frequency channels the UE is supposed to receive. This will obviously lead to extra overhead.